Periodontal disease refers to a state with a disorder in any of gingiva, alveolar bone, cementum and periodontal membrane, which are periodontal tissues that support the teeth, and representative diseases include chronic marginal periodontitis and the like. Periodontal disease is a lifestyle-related disease found in about 80% of the adults, and developed by bacterial infection.
It has been suggested that once periodontal bacteria per se, toxic substances produced thereby, or leukocytes resistant to these bacteria or substances such as cytokine, prostaglandin and the like, which are released from leukocytes, enter the blood and are delivered to the organs in the body, they provide various undesirable influences on the body. For example, diabetes patients can easily have periodontal diseases and a periodontal disease may aggravate diabetes. In addition, it has been confirmed that periodontal disease is a high risk factor of heart diseases caused by arteriosclerosis, and that pregnant women with periodontal disease often have preterm delivery of low birth weight baby and the like.
Thus, since periodontal disease not only causes loss of teeth but is deeply involved in the onset and aggravation of a serious, possibly lethal medical problem, effective means for early diagnosis and treatment, as well as prevention, have been desired.
There are not less than 10 species of causative bacteria of periodontal diseases, of which the most important pathogenic bacterium is Porphyromonas gingivalis (non-patent document 1). P. gingivalis is an anaerobic Gram negative rod-shaped bacterium, which enters into dental plaque and releases enzymes such as protease and the like for its own survival. The enzymes cause inflammation of gingiva, and develop gingivitis which is the beginning of a periodontal disease.
The protease produced by P. gingivalis includes plural molecular species. Particularly, trypsin-like cysteine proteases (gingipain; HRgpA, RgpB, Kgp) are major enzymes produced by this bacterium, and various researches thereof have been made (non-patent documents 2 and 3). Through such researches, it has been suggested that gingipain plays a key role in the maintenance, growth and infection processes of P. gingivalis, and strenuous attempts have been made to develop inhibitors of those proteases for the purpose of treating and preventing periodontal diseases (patent documents 1-4, non-patent documents 4-7). However, the target protein of gingipain in the body is yet to be clarified and information is insufficient for, for example, the research and development and the like of a substrate mimicking inhibitor. Moreover, whether or not protein degradation products due to gingipain are involved in the onset or aggravation of periodontal diseases, or concurrence of systemic complications has not at all been elucidated yet.
On the other hand, as the situation stands, the only highly reliable method for understanding the state of periodontal diseases is measurement of the depth of gingival sulcus (periodontal pocket) by a dental expert (CPI test). For diagnosis of periodontal diseases using a biomarker, detection of bacterium-derived components, blood protein, inflammation related component and the like present in periodontal pocket and saliva has been proposed and, for example, an examination kit for detection of α1-antitrypsin (blood protein) and lactoferrin (inflammation-related component) in interdental liquid and the like are commercially available. In addition, it has been reported that detection of occult blood, alkaline phosphatase (bacterium-derived component), and leukocyte esterase (inflammation-related component) in saliva is effective for risk prediction of periodontal diseases. However, a biomarker permitting an early-stage and certain diagnosis of periodontal diseases has not been found, and there is almost no report relating to a blood biomarker.
In the meantime, with the progress of proteomics research including comprehensive analysis of protein expressed in vivo, a novel biomarker has been actively searched for by utilizing proteomics. Particularly, a study for search of a series of protein degradation products, which emerge in a certain disease in the disease specific manner due to the degradation of a particular target protein by a particular protease, as a biomarker of the disease is called degradomics, and is attracting attention not only as diagnosis of the disease but also as a search means for a new treatment target.
Keratin is a protein constituting an intermediate filament, which is a cytoskeleton of epithelial cells. In stratum corneum tissues such as nail, hair and the like, epithelial cells die by being filled with intermediate filaments consisting of specific keratin called rigid keratin and stiffen. Also in epithelial cells free of cornification such as mucosa and the like, keratin (cytokeratin) plays an important role as a protein constituting the intermediate filament, and the sheet structure of epithelial tissue maintains mechanical strength by the keratin fiber.
However, the relationship between periodontal diseases and periodontal bacterial enzymes and keratin has not been reported.